This invention relates generally to tunable surface acoustic wave (SAW) technology and pertains more particularly to monolithically integrated tunable SAW (MITSAW) devices having a ZnO/MgxZn1xe2x88x92xO quantum well and sensors employing ZnO based MITSAW devices.
Basic SAW Technology
SAW devices have been widely used for signal processing since 1964, when the interdigital transducer (IDT) was introduced. The basic principle of a SAW device is to apply an input IDT and an output IDT in mutually spaced relation to a piezoelectric member, to apply an electrical signal to the input IDT, thereby causing a surface acoustic wave to propagate in the piezoelectric member, and to obtain the electrical signal generated in the output IDT by the propagated surface acoustic wave. The time for the propagated wave to travel from its generation at the input IDT to its arrival at the output IDT constitutes a time delay and the piezoelectric member constitutes a delay path.
SAW Sensors
An important application of surface acoustic wave (SAW) devices is in the field of chemical and biochemical sensing. Surface acoustic waves are very sensitive to changes in the surface properties, including mass loading, conductivity, stress, or viscosity in liquid. Acoustic wave chemical and biochemical sensors have been popular and successfully used in military and commercial applications.
For chemical/biochemical sensing applications, the surface of the delay path (piezoelectric member) is coated with a chemically selective coating which bonds with the target chemical. This delay line is used in the feedback path of an oscillator circuit.
Known SAW sensors have several drawbacks. They are used in the xe2x80x9ctraditionalxe2x80x9d approach, where a single sensor is functionalized with a chemically sensitive material. While this approach is successful when there are a few chemicals present in the environment, its success depends on the availability of highly specific materials, such as suitable biological or molecular-recognition complexes. The mass-sensitive SAW devices can not differentiate between physisorbed and molecularly complexed mass on the surface of a device, unless a secondary perturbation such as a change in mechanical or electrical properties accompanies the mass change. SAW sensor arrays with different chemically independent selective coatings on each sensor have been used for sensing in environments containing many chemicals.
Tunable SAW Technology Using Quantum Well
SAW devices have been widely used for signal processing since 1964, when the interdigital transducer (IDT) was invented. The basic principle of the SAW devices is to use piezoelectric materials to convert an electrical signal into a mechanical wave in the launching transducer, and, conversely, to convert the mechanical wave into an electrical signal at the receiver transducer. An important parameter of a piezoelectric material is the electromechanical coupling coefficient, which is a measure of the conversion strength between electrical energy and mechanical energy. For SAW devices, the coupling coefficient is related to the difference of the metallized (i.e., short-circuited or infinite conductivity) and free surface (i.e., open circuit) velocities.
A problem in SAW technology to date has been the lack of tunability of acoustic velocity, which would allow tuning of the center frequency of the SAW filters. A conductive element near the piezoelectric surface changes the acoustic velocity by coupling with the electric fields of the acoustic wave. Ideally, tunability of the acoustic velocity is limited by the electromechanical coupling coefficient of the piezoelectric material. Early attempts include the use of a semiconductor film in close proximity to the piezoelectric surface. The variable finite conductivity of the semiconductor interacts with the electric fields associated with the acoustic wave, and slows the wave. An improved approach is to use a two dimension electron system (2DEG) to tune the acoustic velocity. An initial demonstration used GaAs/AlxGa1xe2x88x92xAs quantum well. As the piezoelectric coupling of GaAs is very small, the reported tunability range was  less than 0.1%. An alternative hybrid GaAsxe2x80x94LiNbO3 device where the 2DEG was formed in a GaAs quantum well, which was epitaxially lifted off, and bonded to the LiNbO3 substrate. The effective coupling coefficient of this structure was reported to be 3.5% and a velocity tunability of 1.5% was reported. However, the epitaxial lift-off technology is very complicated, with low yields and poor reliability; therefore, it is unsuitable for commercial applications.
Zinc Oxide and its Related Compounds
Zinc oxide is a versatile semiconductor material, with a wide and direct energy band gap (approximately 3.3 eV at room temperature). It has an exciton binding energy (Eb) of 60 meV, which is 2.4 times the thermal energy at room temperature. The large Eb implies that electron-hole pairs are well bound even at room temperature, and efficient radiative recombination is possible if non-radiative recombination sites caused by crystal defects can be reduced by improving the quality of the film. Recently, ZnO has been used for visible-blind UV photodetectors. Optically pumped laser emission has been observed in ZnO films. This opens up the possibility of developing UV lasers from ZnO films. ZnO based ternary alloys, MgxZn1xe2x88x92xO, have been demonstrated, allowing the band gap to be extended up to 4.0 eV. In comparison with other wide band gap semiconductors, ZnO can be grown in the 300 degrees centigrade to 450 degrees centigrade range, hundreds of degrees lower than GaN.
ZnO films have recently been used as the substrate or buffer layer for the growth of GaN based optoelectronic devices. The lattice mismatch between GaN and ZnO is relatively small, which makes growth of high quality films possible. ZnO/GaN heterostructures have been used for hybrid optoelectronic devices. GaN films grown on high quality ZnO buffer layers (grown on Cxe2x80x94(Al2O3) have been observed to have better structural properties compared to GaN films grown on sapphire and SiC.
ZnO is well known as a piezoelectric material used in bulk acoustic wave (BAW) and surface acoustic wave (SAW) delay lines, filters and resonators in wireless communication and signal processing. ZnO thin films have been used in conjunction with low loss high acoustic velocity substrates, such as sapphire (Al2O3) and diamond; with semiconductors, such as Si, GaAs and InP; and with low coupling coefficient piezoelectric materials, such as quartz. ZnO thin films deposited on GaAs and on InP are also used for acousto-optic modulators.
MOCVD Technology
The key issue for high performance, thin film ZnO based SAW device fabrication is the control of the film quality. Many growth technologies have been used to grow ZnO films. Among them, MOCVD (metal organic chemical vapor deposition) technology offers the advantages of high quality epitaxial growth on large area substrates in a production scale.
Applicants herein have used an MOCVD system with a rotating disc reactor chamber. ZnO epitaxial films are grown, using DEZn as the zinc precursor and oxygen as the oxidizer. The gas phase reaction between DEZN and oxygen can occur at room temperature and results in particulate formation, which degrades ZnO film properties, including surface morphology and crystallinity. In order to minimize the gas phase reaction, the MOCVD reactor is designed to have a flow-top configuration with high nitrogen push flow. DEZn and oxygen are introduced into the reactor separately. The substrate is rotated at high speed for improving thickness uniformity.
The present invention has a primary object to provide a ZnO based tunable SAW, preferably monolithically integrated tunable SAW (MITSAW) technology.
More particularly, an object of the invention is to provide SAW sensors having a tunable feature, being able to operate in multiple acoustic wave modes and UV range, and having improved manufacturability.
In attaining these and other objects, the invention provides a SAW device built on piezoelectric ZnO film integrated with a ZnO and MgxZn1xe2x88x92xO quantum well structure.
In one form, the SAW device of the invention includes a piezoelectric member, input and output IDTs disposed on a surface of the piezoelectric member, and a quantum well structure engaged with the piezoelectric member.
The piezoelectric member is comprised of zinc oxide. The quantum well structure is composed of the binary semiconductor of zinc oxide and the ternary compound semiconductor of magnesium zinc oxide (MgxZn1xe2x88x92xO). A substrate is provided for growth of the piezoelectric member and is comprised of R-plane sapphire.
The SAW device is built using R-plane sapphire substrate instead of the popular C-plane sapphire, which offers unique advantages; (i) the c-axis of the ZnO film in the ZnO/Rxe2x80x94(Al2O3) material system is in-plane, resulting with electrical, piezoelectric and optical anisotropy for novel applications; (ii) certain wave modes in the ZnO/Rxe2x80x94(Al2O3) material system have large coupling coefficients and low loss compared to the GaAs/AlxGa1xe2x88x92xAs material system, which significantly enhances the tunability of the acoustic velocity; and (iii) lattice mismatch between ZnO and R-plane sapphire is less than that between ZnO and C-plane sapphire, resulting in high quality ZnO thin films.
In another form, the tunable and multi-mode SAW sensors of the invention include a piezoelectric ZnO member, input and output IDTs disposed on a first surface of the piezoelectric ZnO member, and a quantum well structure disposed beneath a second surface of the piezoelectric Zno member, the quantum well structure including a binary compound semiconductor of ZnO and a ternary compound of MgxZn1xe2x88x92xO. The substrate is comprised of R-plane sapphire.
R-plane sapphire is chosen instead of the popular C-plane substrate, as this substrate provides in-plane anisotropy in the ZnO layer. Rayleigh waves, which are excited parallel to the C-axis, are used for gas-phase sensing, and Love waves, which are excited perpendicular to the C-axis, are used for liquid-phase sensing.
The high coupling coefficients offered by the ZnO/R-plane sapphire system allows velocity tuning larger than 1%. The sensor is xe2x80x9cresetablexe2x80x9d through this tuning mechanism, as operation of the sensor begins at the lower end of the velocity range. The velocity can then be returned to its original value by changing the 2DEG density through reducing the gate voltage. Multiple acoustic wave modes are induced in the layered structure in the high frequency ranges. The responses of the individual wave modes can be selected by adjusting matching networks at the input and output transducers (IDTs).
The sensor can also operate in a UV optical mode. Chemical and biochemical molecules have distinct signatures in the UV spectrum, allowing for accurate identification of the species absorbed by the sensitive coating. In the dual mode sensor operation, the sensor is used as an optical detector for identification of the absorbed species. The SAW response is used to monitor the absorption rate and amount.
In a further embodiment, the invention includes the above described sensor as one channel of a two channel sensor. The other channel includes structure which is identical to the one channel, except for the sensing coating. The two channels are placed in the feedback path of two identical oscillators, and the output of the circuit is the difference of the two frequencies. With this arrangement, the SAW sensor increases the sensing accuracy by eliminating response due to changes in the environment other than the monitored chemical species.
The foregoing and other objects and features of the invention will be further understood from the following detailed description of the preferred embodiments and practices and from the drawings.